|
BIOL
4160
Evolution
Phil Ganter
301 Harned Hall
963-5782 |
This sea
anemone is a lone individual but may close relatives are colonial
in such a way as to challenge our understanding of the terms
"organism" and "individual", which are essential
to the ideas of conflict
and cooperation |
Cooperation and Conflict
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Cooperation and Social
Interaction
Conflict and cooperation are possible outcomes
from interactions between: genes, individuals, populations, species,
or even communities
- Cooperation and conflict are opposite ends of
a spectrum of interactions
- Conflict is at the heart of many evolutionary
models
- Natural or any other sort of selection can be
seen as a conflict between genotypes for survival
It has been more difficult to model a role for
cooperation
- Cooperation within a group (group selection,
if persistent over generations) has costs and benefits for all involved
- The difficulty arises due to Altruism, when individuals
bear the cost (individual level selection) but all members of the group
gain
the
benefit
- Cooperative situations are vulnerable to Cheaters (also called Defectors), group members that benefit from the cooperation
but do not contribute their
share of the cost
The textbook sees little role in nature for group
selection
- Example from the text demonstrates that cheaters
prosper (here, cheater cells that fail to contribute to the vegetative
parts of the slime mold's reproductive system and preferentially contribute
to the cells that give rise to spores)
- The question that is not asked is this - if
cheaters prosper, why then do slime molds still make the vegetative portion
of the reproductive
apparatus?
- If all make spores and none make stalk, why are
there stalks in nature? Could it be that group selection is more
important than standard evolutionary theory currently view it as being? I
am reminded of the first models of insect flight which demonstrated that
bees could not fly.
Evolutionary
Stable Strategies (ESS)
This is a situation (really not appropriate
to call it a strategy as this implies a goal and is teleological) in
which no mutant can increase its frequency due to natural selection alone
ESS can often provide explanations of why a mixture
of "strategies" persists in a population, as the
stable conditions often includes a mixture, none of which can
dominate but all of which, as a
whole, cannot be outdone by any single strategy
- It is the optimal phenotype for the conditions
affecting fitness
- ESS is derived from game theory and has been
mostly used to compare possible sets of behaviors
- Prisoner's
Dilemma is often used as an example
- From Wikipedia: "Two
suspects are arrested by the police. The police have insufficient
evidence for a conviction, and, having separated both prisoners,
visit each of them to offer the same deal. If one testifies
(defects) for the prosecution against the other and the other
remains silent, the betrayer goes free and the silent accomplice
receives the full 10-year sentence. If both remain
silent, both prisoners are sentenced to only six months in
jail for a minor charge. If each betrays the other,
each receives a five-year sentence. Each prisoner must
choose to betray the other or to remain silent. Each
one is assured that the other would not know about the betrayal
before the end of the investigation. How should the
prisoners act"
- It pits the costs and benefits
from cooperating versus those from "defecting" -
i. e. refusing to cooperate.
- In the simplest cases, defecting
is the best strategy
- There are countless variations
on this game (look to the book's explanation of the famous
Hawks and Doves situation for an example of a case where
a mixed strategy is an ESS)
- Iterated
Prisoner's Dilemma alters
the game by running it again and again and assessing the long-term
return from each strategy (ESSs differ if, for instance, the
game is for a pre-determined number of iterations or if the
number of iterations is not known at the outset or if there
are infinite iterations)
- In these games, total
cooperation has the best payoff is there is no defection
(ESS is called a Pareto optimum)
- If cheaters are present,
the iterated form of the game allows one to identify
and punish cheaters and allows for may possible mixed
strategies
- Tit-for-tat (respond to
others as they have responded to you - cooperate
with cooperators and defect from defectors) is an
example of a mixed strategy that can be an ESS under
some condition
Four general types of explanation of cooperation
that do not rely on Group Selection
- Kin Selection -
when the organism donating the resource is donating it to kin who will
indirectly spread the donor's genes
- Direct Individual
Advantage (note "direct" added
to the book's terminology) - when the cooperation is not a
cost but a direct benefit (or the benefits outweigh the costs) for
the individual performing the altruistic act (in this case, the behavior
is only apparently altruistic)
- Manipulation -
when the organism donating resource is forced ("manipulated")
to donate
- Reciprocation (a
specific example of the Transactional Model
of Reproductive Skew) - when cooperation brings just
enough added reproductive opportunity to subordinates that the do better
than if they did not cooperate
- if the interaction is not between dominant
and subordinate individuals, then this interaction is called Reciprocal
Altruism, where individuals donate if they are also getting benefits
(for foraging flocks of birds, those watching for predators (sentinels)
will warn others of their approach and expect the same from others
who watch when the current sentinels are eating)
- notice that, in this case, there need
be no close familial relationship between the members of a flock
- when there is no relatedness, the
benefits for both sides are equal for both
- as the degree of relatedness increases,
the immediate benefits may become more and more skewed (one member
of the transaction gets most of benefits or pays less of costs)
- This is because the donor gets not
only some direct benefits, but also will get indirect benefits
when the recipient of the benefits reproduces and passes on some
of the donor's genes because the donor and recipient are related
- Kin Selection is really a case of
the Transactional Model when relatedness is close and Reciprocal
Altruism is at the opposite end of the spectrum (no relatedness)
Cooperation and Conflict
in Families
- Parents invest (potentially limiting) resources
in their offspring
- Many animals only invest the cost of
gamete production, which is often greater (per gamete and total)
for females
- Some animals invest resources after
fertilization (guarding young, feeding young) which increases the
cost of each offspring
- Promiscuous Mating (Polygamy)
- no parental care is invested and one or both sexes may mate with multiple
partners
- When females provide care and males
do not, the males are promiscuous (Polygyny)
- When males provide care (some frogs,
birds and fish), the females are promiscuous (Polyandry)
- Monogamy -
Some animals (birds and mammals and even some insects) form pair-bonds and
generally mate only with the other member of the pair
- Extra-pair matings are usually present
(strict monogamy is rare)
- Pair-bonds often only last for a single
reproductive season for many long-lived animals (new parings each
season)
- Conflicts arising from Parental Care
- The asymmetry of costs to parents from
reproduction leads to conflict between males and females over which
will provide care
- Since the benefits of successful
care are shared equally by both parents, the sex that experiences
less of a cost (in terms of lost reproductive opportunity)
in providing the care will be the one actually providing
it
- If care is only guarding
the young, males may pay less of a price if they can
still find opportunities to mate with other females
- If feeding is also provided,
males may desert because they lose the mating opportunities
due to the extra time needed to forage for the young
- If male and female losses
are balanced at some intermediate point, an ESS with
both sexes providing care may be possible
- Parent-Offspring
Conflict
- A second asymmetry in parenting
is comes from the difference in relatedness between siblings
and parents
- Offspring are completely related
to themselves (r = 1.0), usually half related to their siblings
(r = 0,5) and half related to each parent (r = 0.5)
- Thus, when a parent distributes
food, there is no reason for favoring any particular
offspring based on relatedness
- Offspring are more related to
themselves than to siblings and so their interests are best
served if the parent devotes more resource to them than to
siblings, even though the parent has no reason to do this
- This can lead to conflict between
parent and offspring about distribution of resources
- Example -
- Haplodiploidy means that
a female is half related to her female offspring and
completely related to her male offspring
- Haplodiploidy means that
female siblings are 3/4ths related but only half related
(r = 0.5) related to their male siblings
- Thus, females will want
to devote more resources to male offspring than the
workers want, since they are more related to sisters
than brothers
- Paternity -
the third asymmetry in parenting that needs attention
- Females are usually able to identify
which young are theirs (carry their genes) but males are often
unable to tell this
- Male behaviors have developed that help
ensure the paternity of offspring
- Mate
Guarding -
males prevent subsequent matings with other males so
that they are guaranteed that a female's eggs are fertilized
by their sperm
- Infanticide of
offspring not likely to be their own
- (not the only reason for infanticide
as instances of infanticide occur when parents responsible
for care do not have resources for all of the young)
- Siblicide -
to continue the theme from the above, many infants die not as a result
of infanticide (adults killing offspring) but through siblicide (sibs
killing sibs)
- When resources are plentiful, sibs benefit
one another through inclusive fitness
- When resources are scarce, in individual will
pass on more of its genes that will its sib (normally relatedness is
no more than 0.75) and so sibs may compete to the death for the available
resources
- Some sharks eat siblings while still
in their mother's uterus, which may be a case of limited resource
or it might be an extension of Oophagy (where egg production continues
as embryos develop and the developing sharks eat the eggs) as a
means of feeding the developing young.
Genetic Conflict
Some genes code for proteins that enable them
to be transmitted to an individual's offspring at a greater than expected
rate, acting in its own behalf (a Selfish Gene)
- When the selfish allele is in the heterozygous
state, it is expected to be in only 50% of that organism's gametes, but
some selfish genes can increase their transmission rate
- Alleles that are selfish may often lower the
overall fitness of an organism, which may be restored by an allele at a
separate locus (a Restorer Gene)
- The antagonistic actions of the restorer and
selfish genes are en example of Genetic Conflict
In a species where the microgamete (sperm, pollen)
does not contribute cytoplasm to the zygote, all cytoplasmic inheritance
is through the macrogamete (egg)
- In a hermaphrodite, any cytoplasmic gene (mitochondrial,
chloroplast) that can increase female effort (more eggs - usually at
the expense of making
sperm or
pollen) is favored over an allele which does not
In a book called Genes in Conflict, Austin
Burt and Robert Trivers examined the ways in which genetic conflict has
arisen. They identified ten general mechanisms:
- Autosomal Gamete Killers (the t-allele is an
example)
- Selfish Sex Chromosomes
- Genomic Imprinting - Alteration of an allele (methylation is usually
at the basis of the alteration) such that it is expressed only if inherited
from a particular parent.
- The success of an allele might depend on which parent it came from,
setting up conflict
- In species where a female's offspring have multiple paternity,
an allele from a particular male parent is found in those
offspring sired
by that male and should encourage the success of
offspring from that male at
the expense
of those offspring sired by other males
- In this species, the same allele
transmitted by the female, has no interest in promoting some
offspring over
others
- Genomic imprinting allows the allele to be expressed ONLY IF IT COMES
FROM THE FATHER in this situation and maximizes the benefit from
skewing maternal effort
- Selfish Mitochondrial DNA
- Selfish Gene Conversion
- Transposable Elements
- Female Drive and Selfish Centromeres
- B Chromosomes
- Genomic Exclusion
- Selfish Cell Lineages
I might add that viruses are related to transposable elements and can be
considered, in some ways, the ultimate examples of selfish DNA
Parasitism, Commensalism,
Mutualism and Levels of Organization
This section centers on the question of individuality,
although there is little in the title to suggest this
- We take our biological individuality for granted
and tend to confine questions about individuality to those pertaining to
our participation in social groups
- In evolution, the very question of individuality
is constantly in question
- Consider our cells - does the presence of mitochondria
mean that each eukaryotic cell is a cooperative group?
- In evolution, individuals are the units of the evolving population (we
speak of individual alleles in geneic level selection, individual organisms
in organism-level selection, etc).
- What is an organism? Are a group of colonial
polyps functioning as a single unit each organisms or is the entire unit
the organism?
The textbook opens this discussion in a very interesting
way by considering the impact of endosymbionts (cells of a different species
living in the body of individuals of another species)
- Endosymbionts can
be Parasites (one
species benefits and the other is harmed), Commensals (one
species benefits and the other is not affected) or Mutualists (both
species
benefit)
- Cooperation in this system can occur between host and endosymbiont
(mutualism)
- Conflict can arise between host and endosymbiont (how many endosymbionts
in the host?) and between different strains of the endosymbiont (which
will dominate in hosts with both strains)
- Endosymbionts are usually Vertically transmitted
(passed from mother to offspring) but sometimes are Horizontally transmitted
(offspring not born with the endosymbiont and must acquire it from another,
not necessisarily related host) - many situations include both forms
of transmission
Coevolution is evolution in both host and symbiont
in which the other species is the selective force
- Parasites evolving to evade host defenses or changes
in both endosymbiont and host to reduce the impact of the endosymbiont on
the host are examples
- When transmission is strictly (or almost strictly)
vertical, the success of the endosymbiont is dependent on the success of
the host in which they reside
- Harm to the host should be reduced in this case,
whether the endosymbiont is parasitic or mutualistic
When there is genetic variation among the endosymbionts,
then obligate vertical transmission leads to something very like group selection
- Each population of endosymbionts within an individual
constitute a group
- Some
groups will benefit their hosts more and, if they will increase in the
endosymbiont population as their hosts increase in the host population
- Cheaters within the group (mutations that harm
the host) can not win as their groups will be confined to groups in less
successful hosts
- The textbook does not refer to this as group selection
but introduces a new level of biological organization so that the the combined
host-symbiont is the unit of selection (the individual)
Human Cooperation and Conflict
Evolution and ethics
- Thorny issue
- Those who refer to nature to justify
some human behavior often take a very selective view of what is "natural"
- The simple case of "if it's natural,
it's good" (Naturalistic Fallacy) is easy to disparage
- Rape, murder, incest, war, theft, and
numerous other activities universally condemned in humans all have
been selected in some natural situations
- The primacy of competition and predation,
often cited as "nature's way" is only a partial truth at best
- Cooperation, altruism, play, and sharing
are all common species characteristics
Last updated February 2, 2010
